Usually, a liquid crystal display device includes a liquid crystal cell configured such that a liquid crystal component oriented in a predetermined direction is sandwiched between two supporting substrates such as glass, and an optical film such as a polarizing plate, a phase difference film, a brightness improving film. An adhesive is used at the time of lamination of the optical films and adhesion of the optical film to the liquid crystal cell.
The liquid crystal display device is used as a display device of a personal computer, a television set, a car navigation system and the like in a wide range. The liquid crystal display device is used in various environments including such a severe environment as a high temperature and high humidity. Consequently, it is required for an adhesive to have such excellent durability that separation and air bubbles are not likely to occur even in a long-term use. In addition, in such a severe environment as a high temperature and high humidity, the polarizing plate is subjected to a change of dimension due to shrinkage or expansion, thereby stress is caused in the polarizing plate. In case that the stress associated with the change of dimension is not relaxed by the adhesive laminated on the polarizing plate and liquid crystal cell, the residual stress of the polarizing plate becomes inhomogeneous, thus, a liquid crystal display device using a conventional adhesive causes a problem referred to as so-called “white streaks” that are a phenomenon such that a light is leaked from the periphery of the display part so as to be whitened.
For improving the above-mentioned problem, an adhesive has been proposed, that has a stress relaxation property by that a low molecular weight polymer is added to the adhesive composition. For example, an adhesive composition for a polarizing plate that includes a high molecular weight acrylic copolymer, a low molecular weight acrylic copolymer having weight-average molecular weight of not more than 30,000 and a polyfunctional compound has been disclosed (refer to Patent Literature 1). The adhesive composition described in Patent Literature 1 follows the change of dimension of the polarizing plate, thus the occurrence of white streaks can be prevented. However, the adhesive composition described in Patent Literature 1 contains a large amount of the low molecular weight acrylic copolymer having weight-average molecular weight of not more than 30,000, thus in high temperature and high humidity, there is a possibility that air bubbles and separation occur in the adhesive composition.
In addition, an adhesive composition has been disclosed, that includes a mixture of a low glass-transition temperature (Tg) acrylic copolymer having a content of a functional group-containing monomer of not more than 0.5% by weight and a high Tg acrylic copolymer having a content of a functional group-containing monomer of not less than 6% by weight, a crosslinking agent reactive with the functional group and an isocyanate compound, and that has a gel fraction of less than 30% by weight (refer to Patent Literature 2). According to the description of Patent Literature 2, the adhesive composition described in Patent Literature 2 has a configuration that a crosslinking structure is formed in the molecules of the high Tg acrylic copolymer, and the molecules of the low Tg acrylic copolymers are connected to each other by a multimeric complex of the isocyanate compound, thereby the adhesive composition is capable of controlling the movement of the adhesive component in the adhesive layer. However, although the liquid crystal display device using the adhesive composition hardly cause the white streams, the adhesive composition has an insufficient crosslinking density and low cohesion force under high-temperature condition, thus it is difficult to prevent occurrence of separation or air bubbles in an evaluation of durability.
Furthermore, recently, in accordance with the increase in size of the liquid crystal display device, the size to be required of the polarizing plate is becoming also larger. Dimensional change such as shrinkage or expansion of the polarizing plate becomes larger in proportion to the increase in size, thus durability and prevention of white streams are required at higher level. As an adhesive composition that satisfies durability even if used for a liquid crystal display device increased in size, an adhesive composition of which cohesion force is heightened has been disclosed. For example, an adhesive composition for a polarizing plate has been disclosed, that includes a mixture of an acrylic copolymer (A) having a carboxyl-group or an amide group but not having a hydroxyl group and an acrylic copolymer (B) having a carboxyl-group or an amide group and having a hydroxyl group, the copolymer (A) and copolymer (B) being mixed in a weight ratio ((A)/(B)) of 20/80 to 50/50 and an isocyanate compound blended to the mixture (refer to Patent Literature 3). However, although the adhesive composition described in Patent Literature 3 is capable of preventing occurrence of separation and air bubbles under high-temperature and high-humidity conditions, it is deficient in an cohesion force necessary for preventing the white streams at a good level in a liquid crystal display device increased in size, thus the liquid crystal display device increased in size using the adhesive composition cannot perfectly prevent the white streams yet.